Various types of spark plugs for igniting an air-fuel mixture are prepared according to types of internal combustion engines to which the spark plugs are to be mounted. Thus, in, for example, an automobile assembly process, in order to allow a worker to readily select a spark plug of a type corresponding to an engine to which the spark plug is to be mounted, the spark plug bears an identification mark (mark) corresponding to a spark plug type. In order to facilitate checking of the identification mark in the course of or after mounting the spark plug, the identification mark is marked on the rear end surface of a metal terminal. Also, some spark plugs bear a mark indicative of orientation of mounting to an engine (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2004-92410).
Incidentally, since the identification mark is formed, by, for example, printing, on the rear end surface of the metal terminal formed from metal, the identification mark may have a chip, patchy appearance, or a like defect. However, even though the identification mark has a chip or patchy appearance to some extent, since the identification mark is sufficiently identifiable depending on the degree of chipping or patchy appearance, such a defect does not hinder automobile assembly work. Thus, conventionally, after the identification mark is printed, an inspector visually inspects to see whether or not the identification mark is identifiable. However, visual inspection by an inspector consumes time and labor. Also, since the degree of chipping or patchy appearance of the identification mark to be judged identifiable varies from person to person, a worker may fail to identify the identification mark which has been judged identifiable by the inspector. Thus, the inventors of the present invention et al. captured an image of an inspection object surface and tried inspecting the identification mark by use of a publicly known image processing technique.
However, since the rear end surface of the metal terminal is a metal surface and has residual fine irregularities, a captured image is apt to be susceptible to reflected light resulting from irregular reflection of illumination light which illuminates the rear end surface of the metal terminal in capturing an image; thus, image processing has encountered difficulty in identifying the identification mark.